PROJECT SUMMARY/ABSTRACT Epidemiological studies indicate that nursing protects against Type 2 Diabetes (T2D), with risk reductions of 40- 50% in all women and up to 60% in women with a history of gestational diabetes mellitus (GDM). Despite these impressive risk reductions, we know little about the molecular mechanisms underlying the beneficial effects of lactation on diabetes risk in either rodents or humans. We propose that the reduction in risk of T2D is a consequence of changes in fuel and energy metabolism associated with the metabolic demands of milk production. Insulin resistance (IR) occurs during pregnancy and is the earliest abnormality defined in patients at risk of T2D, typically preceding the onset of diabetes, sometimes for long periods of time. Many studies have demonstrated the importance of intracellular tissue TGs in the pathophysiology of IR and T2D and we have now generated preliminary data demonstrating that lactation lowers liver triglyceride content in post-partum mice. Therefore, the central premise and hypothesis of this application is that lactation increases tissue lipid clearance post-pregnancy, which, in turn, improves insulin sensitivity and lowers the risk of T2D in both rodents and women. The goal of this R21 application is to investigate the mechanisms through which lactation prevents diabetes in animal and human studies. We propose 2 specific aims. Aim 1 will examine whether lactation mobilizes tissue triglycerides and improves insulin sensitivity in mice. We will perform detailed analyses of energy expenditure, insulin sensitivity and tissue triglyceride content in post-partum mice allowed to lactate or prevented from lactating. Aim 2 will examine whether lactation improves insulin sensitivity and increases lipid turnover in women. We will recruit 2 cohorts of women with a history of GDM, those who either nurse or bottle feed their babies for 6 months, and will assess glucose metabolism, insulin sensitivity and lipid turnover in each group. Obesity and T2D are important health problems in both the industrialized and developing worlds and up to 50% of women with a history of GDM develop T2D. Understanding the mechanisms that mediate the beneficial effects of lactation may point towards new therapeutic approaches to improve the metabolic health of all women and will help reduce the risk of T2D, especially in those with a history of GDM.